Field of the Invention
The present invention relates to a bypass turbojet for an aircraft, in particular an airplane.
Description of the Related Art
Reducing sound emission from airplanes in the vicinity of airports is a major challenge, and ever stricter standards are being imposed on this topic.
Investigation efforts have identified and quantified the main phenomena responsible for the high noise levels generated by airplanes during landing and takeoff. Roughly speaking, noise can be separated into two major categories, namely airframe noise and engine noise.
Airframe noise is due to interactions between the flow of air and obstacles or irregularities on the surface of the airplane, such as landing gear, leading edge slats, trailing edge flaps, etc.
With a bypass turbojet, engine noise mainly comprises jet noise due to rapid ejection of hot gas by the nozzle, and fan noise due to interactions between the flow of air and the stationary structure, which interactions are generated by the fan airfoils driving the secondary or “bypass” stream.
It should be recalled that a bypass turbojet has a fan wheel carrying blades and rotating within an annular casing. Downstream from the fan, a portion of the air stream is directed to a primary or “core” passage in which a primary stream flows that is compressed by a low-pressure compressor and by a high-pressure compressor in order to feed a combustion chamber, with another portion of the air stream from the fan being directed into a bypass passage into which there extends a set of outlet guide vanes (OGVs).
The sound spectrum from a fan may be subdivided into a tonal portion and a wide band portion of magnitude that has increased with the appearance of turbojets having a very large bypass ratio, i.e. in which the bypass stream is large.
There are many wide band noise sources in a fan, with a dominant source being the interaction between the wake from the fan wheel and the stator, and in particular the OGVs.
In order to minimize the noise generated by the fan, one solution consists in optimizing the design of the guide vanes. Nevertheless, constraints on the efficiency of the engine at cruising speed do not enable the vanes to be best adapted for operation during approach or takeoff.
U.S. Pat. No. 3,730,639 proposes fitting the outer casing of the fan with suction means including an air inlet opening implemented by a slot located upstream from the radially outer ends of the blades of the fan wheel, together with a suction pump.
In this way, a portion of the boundary layer formed on the inside wall of the outer casing is sucked in so as to limit interactions between the boundary layer and the blades of the fan wheel, thereby reducing noise. That solution does not significantly reduce the dominant component of the noise, which is generated by the interaction between the wake of the fan wheel and the OGVs situated downstream therefrom.
Furthermore, the presence of clearance between the casing and the ends of the blades of the fan wheel leads to turbulence being generated by an effect of the flow at the tips being entrained, which turbulence carries a high level of turbulent kinetic energy that, once mixed with the boundary layer on the casing, contributes significantly to the wide band noise of the interaction between the turbulent wake and the OGVs.
A known solution consists in providing means for injecting air into the turbulent zones of the fan. To do that, air-passing channels need to be made in the blades of the fan wheel, thereby making such blades more complicated to manufacture, in particular when they are made out of composite material.